I used this video as a guide. YouTube - 2005-2009 Ford Mustang Speaker Replacement How-To Guide
It is for 05-09, but ours are basically the same. It leaves a few small things out, but is a good enough guide to remove the door panels and rear deck or at least show you if its something you want to get in to.
They also recently posted the entire service manual up at the top of the 2011+ GT page, which has details on removing the interior pieces.
2011 Sterling Gray GT Premium
3.55 | 6MT | Factory 19's | Security Package
Mods: Borla S-type axlebacks, sound, tint, louvers, Epco shift knob, Airaid CAI, BAMA 93 Race tune, Batron shifter and 2 post bracket
The few, the Proud, the 5.0's.....